This invention relates to a pulse compression apparatus for and more ultrasonic image processing, particularly, for examining portions of a body to be diagnosed such as a living body using ultrasonic waves.
In order to examine portions of a body to be diagnosed or a living body, it is desirable to use an ultrasonic pulse diagnostic apparatus which can make a diagnosis using ultrasonic pulses, with a long examining distance and a high distance resolving power. The resolution power of an ultrasonic diagnostic apparatus which employs a pulse reflection method to take the tomographic image of a living body can be divided into the distance resolving power in a direction along the ultrasonic beam and the direction resolving power in a direction perpendicular to the direction of propagation of the ultrasonic beam. The distance resolving power can be enhanced by reducing the pulse width of the transmission waves. In order to reduce the pulse width, the ultrasonic transducer may be designed to have a wide frequency band. For this purpose, for example, the quality factor Q of the ultrasonic transducer is set small by using damping material. However, when the damping material is used to obtain small Q, transmission and reception power loss caused by using the damping material is increased, thereby lowering the transmission and reception sensitivity. Therefore, the damping of ultrasonic waves should be set higher than a preset lower limit. Another method for reducing the pulse width is attained by increasing the operation frequency (fo) of the ultrasonic transducer which can be obtained from the following equation: EQU .DELTA.f=fo/Q
where .DELTA.f is the band width of the ultrasonic transducer. In this case, however, since the attenuation of the ultrasonic waves in the living body rapidly becomes large, the maximum permissible diagnostic distance and the detection sensitivity becomes lower as the operation frequency becomes high. In contrast, the direction resolving power can be enhanced by setting the diameter of the ultrasonic beam small. The diameter of the ultrasonic beam can be made small by making the opening of the ultrasonic transducer large. However, in this case, the focus range is made small and the resolving power is lowered in an area outside the focus range. The direction resolving power can be also be enhanced by increasing the operation frequency with the opening kept small. However, also in this case, the ultrasonic waves are attenuated in the living body, preventing a diagnosis from being made over a long distance. In an ultrasonic diagnostic apparatus such as an ultrasonic endoscope in which the opening of the ultrasonic transducer is limited, it is advisable to use a high operation frequency if the direction sensitivity is satisfied.
With a high operation frequency, the resolving power can be enhanced, but the detection sensitivity will be lowered as described above. Reduction in the detection sensitivity can be compensated for by increasing the transmission power by a corresponding amount. This cannot be put into practice since there are various restrictions including the withstanding voltage of circuit elements in actual circuits. In order to solve such a problem, a pulse compression method (frequency synthesis) which is practically used in radar system and the like can be utilized. The method is briefly explained in the following.
Transmission waves from a radar are subjected to a linear-FM, or chirp, as shown in FIG. 4A and then transmitted. In the receiving mode, radar reflection waves are received by a receiving circuit (matched filter) having such receiving circuit characteristics as shown in FIG. 4B, thus obtaining a compressed reception signal as shown in FIG. 4C. The prior art radar system utilizes a special SAW (Surface Acoustic Wave) filter dealing with surface acoustic waves shown in FIG. 5 in order to attain the receiving circuit characteristics as shown in FIG. 4B. When a receiving signal passes through the matched filter, the amplitude is amplified to .sqroot.T.multidot..DELTA.f times the original value and the pulse width (-3 dB) is compressed by 1/.DELTA.f. Thus, the detection sensitivity is determined by the average transmission power, and the distance resolving power varies with the reciprocal of the frequency band width of the linear FM modulation.
As described above, a pulse compression method is used in the conventional radar system using a SAW filter. However, in an ultrasonic diagnostic apparatus, the operation frequency is high and the frequency band width is large, making it difficult to utilize a SAW filter for the pulse compression technique. Further, the characteristics of the SAW filter are determined by the geometric shape of the electrodes such as the pitch and length of the electrodes and therefore cannot be changed. In an ultrasonic diagnostic apparatus, when a body in which ultrasonic waves are attenuated by a large amount is examined by the pulse reflection method, it is necessary to dynamically change and attain the filter characteristics which can be best suited for the examination. However, with the SAW filter, dynamically variable filter characteristics cannot be attained as described above.